Alteration of central auditory processing of brief stimuli: a review and a neural model

There is evidence that the manner in which the central auditory system processes transient signals can be altered by either of two seemingly unrelated experimental factors, namely, hearing loss, or the presence of a continuous tone of moderate intensity. In particular, altered processing of brief signals in the central auditory system is indicated by either: (i) lower (better) behaviorally measured thresholds for pulsate electrical stimulation of brain-stem auditory nuclei, or (ii) an increased amplitude of some components of the brain stem or cortical potentials evoked by brief acoustic stimuli. A simple two-neuron model which evolved from the above findings and related visual system phenomena was used to evaluate the effects produced by hearing loss or continuous tone. In the model, a "hearing loss" condition, which was represented by less spontaneous neural activity (and, hence, reduced inhibition) relative to a "normal hearing" condition, yielded both increased gain for steady-state stimuli and increased responsiveness for transient stimuli. Thus the model provides two possible, and simultaneously operable, explanations of reduced electrical stimulation thresholds and increased evoked potential amplitude: one based on increased steady-state gain and the other based on increased responsiveness to transient stimuli. A third condition, "normal hearing with a longer excitatory time constant," yielded effects that were similar to those produced by reduced inhibition, thereby suggesting a physiological basis for alteration of psychophysically measured time constants.